Self-locking threaded fasteners

ABSTRACT

A self-locking threaded fastener such as a screw in which locking action is obtained from at least one locking zone, parts of two adjacent turns leaning toward each other and including between them a groove of reduced thread angle. The threads on the product are advantageously formed by rolling with novel dies including a lock forming area in which normal internal support for the flanks of developing thread ridges are removed during the latter part of the thread-rolling operation thereby causing the pressure applied to external flanks to result in leaning of the locking turn portions toward each other.

United States Patent Primary ExaminerMilton S. Mehr Attorneys-W. BigelowHall, Richard A. Wise and Maurice R.

Boiteau ABSTRACT: A self-locking threaded fastener such as a screw inwhich locking action is obtained from at least one locking zone, partsof two adjacent turns leaning toward each other and including betweenthem a groove of reduced thread angle. The threads on the product areadvantageously formed by rolling with novel dies including a lockforming area in which normal internal support for the flanks ofdeveloping thread ridges are removed during the latter part of thethread-rolling operation thereby causing the pressure applied toexternal flanks to result in leaning of the locking turn portions towardeach other.

PATENTEU JAN 41972 sum 1 BF 3 Inve Howard 1 P061? PATENTEU JAN 4 i372SHEET 3 [1F 3 SELF-LOCKING THREADED FASTENERS This is a division ofapplication Ser. No. 731,416, filed May 23, 1968, now U.S. Pat. No.3,530,920.

This invention relates generally to self-locking threaded fasteners andmore particularly to such fastenersin which a locking zone is fonned inthe thread itself without the introduction of added locking elements.

The widespread use of self-locking threaded fasteners in goodsmanufactured in large quantities is highly desirable because suchfasteners very often not only contribute to greater safety in the use ofthe goods but may also add to the reliability and durability of thegoods in areas of the product where safety in their use is not animportant factor. Although the use of self-locking threaded fasteners isgrowing rapidly, there have been a number of reasons which havecontributed to restricting their more extensive use. A major limitingfactor has been that of relatively high installed cost of conventionalself-locking threaded fasteners. In those fasteners which derive theirlocking action from a plastic locking element, several operations arerequired for mounting the locking element on the fastener. On the otherhand when the locking action is obtained by a special thread form in oneor both of two mating threaded members to produce an interference fit,not only are added costs encountered in providing special tooling forthe manufacture of the special thread but also and more importantlyexpenditures are caused by the fact that such fasteners usually have asignificiantly higher installation torque. Under the most adversetolerance conditions, seating torque may reach such a high level thatbolt heads are actually sheared off during installation. At any rate,heavier tools are required for a given screw size and production ratesare lowered thereby further adding to the installed cost.

Another important factor in limiting the more widespread use ofself-locking fasteners is the failure of conventional selflockingfasteners to satisfy the requirements of the environment in which thefasteners must be used or of the normally expected conditions of use.For example, many of the conventional self-locking threaded fastenersare ineffective at temperatures exceeding 300 F. In addition, fewconventional selflocking fasteners are completely reliable whensubjected to severe vibration for extended periods of time.

In addition to adverse environmental conditions of temperature andvibration another detrimental aspect of conventional self-lockingfasteners relates to the manner in which the locking action is produced.Generally in fasteners without added locking elements, the lockingaction is obtained by stressing one or both of the mating elementsbeyond their elastic limits. As a result such fasteners tend to showsubstantial loss of locking quality after limited use. When employedwith mating parts manufactured to standard commercial tolerances, suchfasteners often lose their locking characteristics almost completelywhen their first mating part is replaced by another.

It is accordingly an object of the present invention to provide aself-locking threaded fastener in which the thread including a lockingzone may be produced economically on standard commercially availablethread forming machines employing standard tools requiring a minimum ofmodification.

Another object is to produce self-locking screws at speeds comparable tothose which would be achieved in processing standard nonlocking screwsof comparable size and material.

Yet another object is to provide a locking thread structure adapted toretaining its locking action after repeated reuse and in spite of aninterchange of mating part.

In the achievement of the foregoing objects a feature of the inventionresides in the formation of a thread of a special locking form in apredetermined zone while threads of standard form may be produced alongthe remainder of the length of the fastener. In a screw according to apreferred form of the invention, for example, the thread is locallymodified in its form to produce a locking action for an extent rangingbetween a small fraction of a thread turn up to a half turn which willbe referred to as an elemental locking zone. It will be realized,however, that two elemental locking zones may be located end to end on ascrew and thusprovide acomposite locking zone extending 360 about theaxis of the screw.

The modification of the thread in the lockingzone while not completelyuniform throughout its extent, is generally characterized by severalvariations from the standard thread. In a screw, the thread in thelocking zone is formed with a reduced thread angle, a shorteneddedendum, a broadened crest width and an increased width across theflanks. As a result, when such a screw is engaged in a standard threadedopening or .nut, there is a controlled localized strain within theelastic limit of the material in the screw. The thread form in thelocking zone yields an interfering fit primarily in the flank area as itadapts to the standard thread form of the mating part. The degree ofdeviation between the standard thread form and that in the locking zoneis accurately controlled to maintain the stress within the elastic limitof the screw material. Furthermore, it is possible to confine the strainto the elastic range of the material of the screw even though thestandard threads of the mating part are manufactured to broad commercialtolerances and still obtain repeated locking action superior to thatconventionally attainable.

Another feature of the invention relates to the method by which screwsmay be processed so that the locking zone is formed at the same timethat the threads are rolled without requiring supplementary operations.In the practice of the method a screw blank of standard size is rolledbetween thread rolling dies which may be purchased as standard dies andreadily modified to produce the locking zone. It will first beunderstood that during a conventional thread rolling operation, materialof the screw blank is progressively forced and directed by the serratedsurfaces of the dies from the valleys into the crest to form the thread.However, in practicing the present method at least one of the dies isprovided with a lock forming area in which the material of the screwblank is radially and axially compressed at points separated along theaxis of the screw while the intervening screw blank surfaces, which havealready been partly formed are left free of support and guidance. Thisis accomplished by eliminating from at least one of the dies the supportfor the intervening screw blank surfaces. This is conveniently achievedby removing a single ridge from the die for part of its length. Theresult of this relationship between screw blank and rolling die as theblank travels its last half revolution between the dies, is a preciseand predictable local advance of blank material into the thread groove.

Other features of the invention relate to the modified rolling diesthemselves, and to the extent of the support removed from the dies toproduce a novel screw having a plurality of locking zones atpredetermined relative locations in the finished product.

The foregoing objects and features and numerous advantages of thepresent invention will become more evident from the following detaileddescription of an illustrative embodiment of the invention together witha description of the basic process by which the embodiment is realized,taken in connection with the accompanying drawings in which:

FIG. 1 is a view in side elevation and on an enlarged scale, and with aportion broken away for clarity, of a screw including a locking zoneaccording to the present invention;

FIG. 2 is a plan view of a pair of rolling dies engaging a screw forforming threads including a locking zone in the shank of the screw;

FIG. 3 is a view in perspective of a pair of rolling dies one of whichhas been modified to produce a locking zone on a screw;

FIG. 4 is a view in cross section and on an enlarged scale of therelationship of a standard nut with a screw having a locking zoneaccording to the invention;

FIG. 5 is a view in perspective of a rolling die having a modified lockforming area;

FIG. 6 is a fragmentary view on an enlarged scale showing the lockforming area in the die of FIG. 5;

FIGS. 7, 8 and 9 are progressive views partly in cross section and on anenlarged scale of a rolling die and screw showing in separatedrelationship and exaggerated for clarity the effect on a screw blankduring the last half revolution of rolling by a die modified to form alocking zone on the finished screw;

FIG. 10 is a view showing in separated relationship a normal screwthread profile greatly enlarged and matched with a standard nut profile;and

FIG. 11 is a view similar to FIG. 10 but showing screw threads of alocking zone according to the present invention engaging a standard nutprofile.

Prior to describing the embodiment of the invention as depicted in thedrawings it is considered desirable to explain certain departures fromnormal thread terminology which are adapted for convenience in thisspecification. Normally a single thread is defined as a single-helicalridge of uniform section on the internal or external surface of acylinder, cone in the case of a tapered thread, left by forming acontinuous single helical groove in the appropriate cylindrical surface.Because the locking action in the locking zone according to the presentinvention is derived from localized variations in the form of the ridgeand of the groove and will more clearly appear in an axial section offasteners, it is considered more convenient and less cumbersome to speakof each turn as though it were a separate ridge or groove rather than asa continuation of an adjacent turn. Normal terminology such as crest,root, flank will be retained to refer to parts of a single turn orridge. Another departure from normal thread terminology necessitated bythe nature of the invention is the concept of pitch diameter which isnormally considered to be that of an imaginary cylinder passing throughthreads at such points as to make the width of ridge and groove equal.In this specification, wherever locking action is described, atheoretical basic pitch diameter applicable to normal thread turns isestablished and those thread turns which provide the locking action areconsidered as abnormal at the theoretical pitch diameter.

Turning now to the drawings, particularly FIG. I, there is shown on anenlarged scale, a screw indicated generally at and including a head 22and a threaded shank 26. There are formed on the shank 24 a plurality ofthread turns of normal cross section comprising ridges 26 and grooves28. A locking zone comprises two ridges 30 and an intervening groove 32.The ridges 30 and the groove 32 comprise a single locking zone which maycover from a relatively few degrees to as much as a half turn about theaxis of the screw. A single screw may contain a single locking zone oralternatively a plurality of locking zones having different relativeplacements on the screw in order to suit the requirements of the matingpart which will normally be called simply a nut. A more completeunderstanding of the invention will be obtained, however, from aninitial consideration of the characteristics of the novel thread form ina single-locking zone to be followed by a description of a preferredmethod of forming the singlelocking zone taken together with the toolingmodifications necessary to produce it.

There is shown on a greatly enlarged scale in FIG. 10 the thread profileof a standard screw and in FIG. 1 l a self-locking screw including thelocking zone consisting of the ridges 30 and the groove 32. The threadform in the locking zone of FIG. 11 may be directly compared not onlywith adjacent normal threads on the same screw but also with the profileof the thread of the standard screw. In FIG. 10 the standard threadprofile is designated generally as that of a screw 34 while the screwprofile of FIG. 11 may be considered as an enlargement of aportion ofthe thread on the screw of FIG. 1. Outside the locking zone, the threadprofile of the self-locking screw includes normal ridges 26 generallycomparable in shape to ridges 36 of the screw 34 and grooves 28comparable to standard grooves 38. It will be appreciated that the twothreads whose profiles are depicted in FIGS. 10 and 11 were rolled withdifferent dies on different thread rolling machines and are related onlyin that both are enlargements of 54-20 threads rolled to a class 2tolerance.

In FIGS. 10 and I1 the screws 34 and 24 are shown respec tively withsimilar nuts 40 and 42 which have been drawn to the lead and threadangle of the screw 34 thereby assuming zero thread angle and leaddifference with a normal screw. Pitch lines 44 and 46 have been suppliedto the nuts 40 and 42 respectively and each of the pitch linesrepresents a mean pitch diameter of 0.2193 for a %-20 nut manufacturedto class 2 tolerance. The major and minor diameters correspondingrespectively to root 48 and crests 50 of the nut 40 are respectivelyequal to 0.2500 and 0.2009 as are similarly roots 52 and crests 54 ofthe nut 42. The major diameter has been chosen as the minimumpermissible major diameter whereas the minor diameter is a mean figurefor a class 2 fit in Vo-ZO nuts. In the nut 40 the ridges and groovesare respectively designated by reference characters 56 and 58. In thenut 42 of FIG. 12 the ridges and grooves are respectively designated as60 and 62. A pitch line 64 is shown on the threads of the screw depictedin FIG. 11. Along pitch line 64 the normal or standard portion of thethreads, the ridges 26 and grooves 28 are of equal width. Similarly apitch line 66 has been supplied on the standard screw thread profile 34of FIG. 10. Along the pitch line 66 all the ridges 36 and grooves 38 areof equal width.

Turning now to the angular orientation of the flanks of the ridges, itwill be realized that, in accordance with standard thread tolerancepractice, the thread angle which determines the relative orientations ofthe flanks on the ridges of the screw 34 and the nuts 40 and 42 is veryclose to the basic angle. Since these threads are of national form theangle closely approaches 60, any deviation form 60 together with anylead error in an actual thread being unspecified according to standardsbut rather absorbed in pitch diameter tolerances and allowances forfits. It is also seen that the thread angle between the ridge flanksdefining the normal grooves 28 of the selflocking screw 24 also closelyapproaches the standard 60 angle. However, the thread flanks definingthe locking groove 32 are relatively oriented at an angle somewhat lessthan 60 and consequently a locking action is obtained between the ridges30 at points near their crests contacting the flanks of the related nutcrest 54 as shown in FIG. 11.

It will be appreciated that the showings of FIGS. 10 and 11 do notrepresent conditions which are ever encountered during the engagement ofstandard nuts either with standard screws or with screws having lockingzones according to the present invention. Fundamentally these two FIGS.depict differences in clearances between standard mating threaded pansshown in FIG. 10 in contrast to the clearances obtained between aself-locking screw and standard nut as depicted in FIG. ll when thepitch line of nuts and screws in both FIGS. are separated to the sameextent and the nut profiles are substantially identical. In addition itwill further be appreciated that the angular orientation of the flanksdefining the locking groove 32 is not uniform throughout the lockingzone but in fact continuously varies at different angular positionsabout the axis of the screw. Starting at the entry into the locking zonein one direction the angle of the locking groove 32 varies from a veryslight gradually increasing deviation from the standard 60 angle,increasing the deviation to produce a minimum thread angle comparable tothat of the groove 32 as shown in FIG. 11 and thereafter decreasing to alesser degree of deviation from normal but not returning completely tothe normal thread angle in a gradual manner before dropping off somewhatabruptly to the normal thread angle. Thus, in a right-hand screw, theleading edge of the thread-locking zone, that which first engages thenut, is somewhat more abrupt than the trailing edge. This differencebetween leading and trailing ends of the locking none is a result of thefact that the leading end of the locking zone is in the process of beingfurther deformed when the screw drops out from engagement with therolling dies at the end of a rolling operation. This aspect of theinvention will be more fully appreciated as will the other variationsfrom normal thread profile which produce the locking action, when theflow of screw blank material during the rolling action is explained.

The thread-rolling process and die configuration which will be explainedare also responsible for other deviations from standard thread form inthe locking zone. These other factors include slightly truncated crestson the locking ridges 30 resulting in a slightly broader and lowercrest, a root 68 of the locking groove 32, which is appreciably higheror at a greater radius than the roots of normal threads and a slightlygreater thickness of the locking ridges 30 along the pitch line 64.

In FIG. 4 a single-locking zone encircled at 70 may be considered as theequivalent of the locking zone comprising the ridges 30 and the groove32 in FIG. 11 but will now be described in its locking effect inengagement with a nut 72 rather than by considering, as was done inrelation to FIGS. and 11, the comparison of normal and locking threadprofiles with their pitch lines in spaced relationship d withoutreference to the elastic deformation which takes place during theengagement of a self-locking screw according to the present inventionwith a standard mating part.

As depicted in FIG. 4 an upper locking flank 74 and a lower lockingflank 76 in the zone 70 have been elastically deformed generally toconform to the mating flanks of the nut 72. Under the condition depictedin FIG. 4 in which the screw including the locking zone 70 is free oftension, normal screw flanks 78 tend to be centralized and in spacedrelationship with nut flanks 80 in the axial plane and on the same sideas the locking zone 70. At the same time however the screw is shiftedsomewhat radially in the nut away from the locking zone 70 so that screwflanks 82 diametrically opposite the locking zone 70 are wedged intointimate contact with nut flanks 84. Thus locking action is obtainedfrom the forceful engagement of the flanks 74 and 76 as they areelastically deformed from their original, less than normal angularrelationship to conform to normal thread flanks 80 and from the tightengagement of the opposite screw flanks 82 with the nut flanks 84. Underconditions of screw tension, if it is assumed for example that anarticle of some thickness is tightly gripped between the nut 72 and ahead (not shown) on the screw including the locking zone 70, there is aresultant slight axial shifting of the nut on the screw as the lockingflank 74 and normal flanks 78 correspondingly oriented on the lowersurface of successive screw ridge turns are tightly engaged bycorresponding surfaces of the nut 72 while the screw flanks 78 on theupper surface of normal thread turns are spaced away from correspondingflanks of the nut. The flank 76 tends to return toward its unstressedstate, the crest terminating the flank 76 maintaining its contact withthe related nut flank and thereby contributing substantially to thelocking action. In order still further to improve the elastic clampingaction of the flanks 74 and 76 on the related flanks of the matingthread in the nut 72 locking screws according to the present inventionare, whenever feasible, of a greater hardness than the nut. Thus wnenthe nut is of relatively soft steel the screw may be of a heat treatedalloy steel or a case hardened mild steel of considerably greatersurface hardness than its mating thread. Similarly, as will readilyoccur to those skilled in the metallurgical art, nonferrous screws maybe made harder than the mating threaded members by selecting differentanalyses, by heat treatment, or by work hardening, particularly workhardening inherent in the thread rolling operation.

In FIGS. 2 and 3, there is shown a screw comprising a threaded shank 86and a head 88 in the process of having its threads formed by rollingbetween a pair of dies comprising a stationary die 92 and a movable die94. The dies 92 and 94 are, except as will now be pointed out, standardin every respect, manufactured in large quantities and availablecommercially at low costs. Such commercial dies include work engagingsurfaces which are formed throughout their widths with alternatinggrooves and ridges 96 and 98 respectively spaced at intervals equal tothe pitch of the screw to be produced and with the grooves and ridgesoriented with respect to top surfaces 100 and 102 of the stationary andmovable die at an angle equal to the helix angle of the screw. Startingwith a commercially available pair of thread rolling dies, each having afull complement of grooves 96 and ridges 98, a lock forming area 104 isformed in the work engaging surface of one of the dies by removing afull ridge, generally by grinding since the dies are bought commerciallyin their hardened condition. It is convenient to think of the rollingdies as each having a leading end, that is an end which first engagesthe screw blank and a trailing end which is the last to engage thecompleted screw. Thus in FIGS. 2 and 3 the leading and trailing ends ofthe stationary die 92 are designated respectively by reference numerals106 and 108 while the leading and trailing end of the movable die 94 aredesignated by the reference numerals 110 and 112 respectively. The lockforming area 104 in the die 92 is formed at a distance from the topsurface 100 along the trailing end of the die equal to the distancedesired between the underside of the screw head and the locking zone onthe screw.

The operation of the lock forming area 104 in producing a locking zonesuch as that comprising the ridges 30 and the groove 32 shown in FIG.11, will best be understood by considering the relationship of the lockforming area with the screw threads in the process of formation asdepicted in FIGS. 7 through 9 which shows in an exaggerated manner andin separated relationship for clarity successive relative positions ofthe stationary die 92 and a screw 114 having in an assumed axial planeupper and lower outer flanks 116 and 118 and upper and lower innerflanks 120 and 122. The screw and die conditions depicted in FIG. 9 maybe considered as that obtaining at the time when the screw 114 drops outof engagement with the rolling dies. FIG. 8 depicts the relationshipexisting between the screw 114 and the stationary die 92 onehalf turn ofthe screw before reaching the position of FIG. 9, whereas FIG. 7illustrates the relative positions of screw and stationary die a fullturn before the position of FIG. 9 is reached. Accordingly, the plane ofthe screw depicted in FIG. 7 is the same as that depicted in FIG. 9 butwith the depth of thread shown in FIG. 7 exaggeratedly unforrned or ofshallower than normal depth, for clarity.

Keeping in mind that the stationary die 92 and the movable die 94 arecontinuously being pressed into the body of the screws as it rolls fromthe position of FIG. 7 to that of FIG. 9 and that the threads areaccordingly being correspondingly deepened, it will be realized that asshown in FIG. 7 the outer flank 116 is not being worked and the threadroot terminating this flank, since it is not in engagement with a dieridge 98 remains shallower than the other root of the screw at the sametime and the flank 116 is somewhat thickened by not being worked. Inaddition since the inner flank 120 is fully engaged by a normal ridge 98of the die and the flank 116 is unsupported, die pressure is translatedinto displacement of the flank 116 upwardly.

When the screw 114 is in the position of FIG. 8 the flanks 116, 118, 120and 122 inclusive are in full engagement with a complete complement ofridges on the movable die 94 and the previous growth of the flank 116from not having been worked in the position of FIG 7 together with theupward displacement from the pressure on the flank 120 while the flank116 was unsupported is partially corrected by the movable die. Theflanks 1 l8 and 122 however, have maintained a normal or standardrelationship with the rolling die in both the positions of FIG. 7 andFIG. 8 and are of nonnal thickness and position. In rolling from theposition of FIG. 8 to that of FIG. 9, both of the outer flanks I16 and118 arrive in contact with ridges 98 which define the lock forming area104. However, the flanks 120 and 122 are unsupported so that thepressure on the outer flanks 116 and 118 is translated into adisplacement of the inner flanks 120 and 122 toward each other. Theflank 116 which was not worked in the position of FIG. 7 and onlypartially reformed while the screw was in the position of FIG. 8, causesthe flank 120 deflected from normal position to a slightly greaterextent than the flank 122. Also, because no die ridge is present tooperate upon and support the flanks I20 and 122 in the position of FIG.9, the root between these two flanks is slightly shallower than thenonnal roots and the partial thread turns defined by the flanks 118 and122 and 120 and 116 tend to have a somewhat greater thickness than thenormal threads along their pitch line. This increased thickness as wellas the increased root radius between the flanks 120 and 122 may becontrolled accurately by die adjustment in the thread rolling machine.The thread depth as measured radially from the root of the lockinggroove to the crests of the locking ridges is generally greater than 90percent of the depth of normal threads in the same screw.

A number of characteristics of locking threads according to the presentinvention will now be apparent in view of the preferred method by whichthe locking zone is produced and of the tooling employed in itsproduction. It will first be appreciated that as the screw blankprogresses from the leading end 106 to the trailing end 108 of thestationary die, it typically completes 4 to 6 revolutions in acounterclockwise direction as seen in FIG. 2. From the start of theengagement of the screw blank between the dies, as the screw progressesand the thread on its shank is being formed to a greater depth,potential locking zones are continuously being formed and subsequentlybeing reformed into normal threads. As a result, the only locking zonein the finished screw is that at the predetermined level of the lockforming area in the last half revolution before the completed screwpasses the trailing end 108. The fact that the screw is rotating in acounterclockwise direction as viewed in FIG. 12 causes a slightly moreabrupt return to normal thread sections in the area of a radial plane onthe screw which coincides at the time of release with the trailing end108 of the stationary die 92. This factor is minimized however by thefact that standard commercial rolling dies are normally tapered awayfrom the flat work engaging surface for a distance equal toapproximately oneeighth inch at the end.

Another characteristic which has been mentioned is that the root betweenthe flanks 120 and 122 as shown in FIG. 9 generally tends to be slightlyhigher than normal thread roots. Ample clearance is provided howeverbetween the higher than normal root and the minor diameter of nuts whichgenerally provide 75 percent or less of full thread engagement. Finallybecause the two locking ridges, the first defined by the flanks 116 and120 and the second by the flanks 122 and 118 are finally formed withoutconfinement by an intervening die ridge 98, the crests of these tworidges tend to be slightly lower and broader than normal. Neither theincreased breadth nor the slight truncation appreciably changes the fitof the screw 114 in a standard nut. The locking action is producedlargely by the localized pressure applied by the flanks 120 and 122which are relatively oriented at less than a normal thread angle, beingelastically deformed by engagement with normally oriented flanks of themating thread. The tendency of the normal mating thread to reform theangle between the flanks 120 and 122 is resisted by the elasticity ofthe material and limited by the fit of the normal screw threads with themating part. Accordingly, upon disengagement from a mating thread theflanks 120 and 122 resume a position very close to their originalas-rolled position and therefore maintain their locking ability underthe most severe reusal tests.

It has been found not only that a locking zone may be formed in a screwduring the original rolling of the thread according to the presentinvention but also that screws having normal rolled threads may beprocessed with dies having a lock forming area such as that alreadydescribed to produce a screw having a lock forming zone characterized byselflocking characteristics very similar to those obtained in a screw inwhich the locking zone is formed at the same time that the normalthreads are originally rolled. Because a screw during a normal threadrolling operation is in very high compression along a line perpendicularto the work engaging surfaces of the die, the screw, while being rolled,assumes a slightly ovoid cross section which turns circular upon releasefrom the dies. it is for this reason that a normal or standard screw maybe rerolled to produce a screw having a locking zone, the differencebetween the minor axis of the stressed screw and the diameter of theunstressed screw essentially providing the material for the formation ofthe locking zone.

In the foregoing description of tools and processes for forming lockingzOnes on screws, the removal of a ridge extending the entire length ofone of the rolling dies has been suggested. A useful alternative forpracticing the invention is depicted in FIGS. 5 and 6 in which astationary thread rolling die 130 having a leading end 132 and atrailing end 134 respectively comparable to the ends 106 and 108 of thedie 92, is modified to provide near the trailing end a lock forming areaof limited longitudinal extent. The lock forming area indicated at 136is formed by grinding away part of a single ridge but only for a lengthequal to one-half or even less of the pitch circumference of the screwto be operated upon. lt will be appreciated from reviewing thedescription of successive relationships of the screw 11-! and the die 92flow of the material of the screw 114, that a screw produced by the die130 is characterized in its locking zone by crests more nearlyapproaching normal height and form, a root of more normal depth and athickness at the pitch line of the threads more closely approachingstandard values. However, the product of the die 130 exhibits a lockingthread groove of reduced thread angle at a point corresponding to thelock forming area 136 and such a screw has been found useful forapplications not subject to extreme conditions of vibration or forcestending to disengage the screw from its mating part. In addition, byreducing the length of the locking area 136 to a distance equal tosomething less than half the circumference of the screw, for exampleone-quarter the circumference of the screw, an additional element isintroduced in the control of installation and removal torque. As aresult the basic configuration of the locking area 136, varied in itslength and coupled with the relative adjustment of the dies in therolling machine, provides an effective control of installation andwithdrawal torque of the product to suit different environments.

In the foregoing descriptions of the locking zones 104 and 126 each hasbeen described as produced by the complete removal of a ridge on one ofthe rolling dies, in the case of the area 104 by removing the ridge forthe entire length of the die 92 and in the case of the area 136 byremoval of the entire ridge for a part of the length of the die 130. Itwill be appreciated however from the description of the lock formingaction as depicted in FIGS. 7 to 9 that the displacement of the flanksand 122 from the standard to the locking position is produced byremoving from between these two flanks the normal die support providedby a ridge 98 such as that which acts on the other threads of the screw.It is accordingly possible and within the contemplation of the presentinvention to employ a die having a lock forming area in which the wholeridge cross section has not been removed but rather in which the ridgehas been thinned along its flanks. This may readily be accomplishedstarting with a standard thread rolling die and grinding, for example0.00l, from each of the flanks of the ridge instead of removing theridge to produce the lock forming area. While this procedure is moretime consuming, requires greater accuracy and is accordingly moreexpensive, it does permit the exercise of an additional area of controlover the shape of the locking zone in the screw and of its lockingcharacteristics.

The product, tooling, and method according to the present invention havethus far been illustrated and discussed with specific reference to pairsof rolling dies in which a single die is modified by the provision of asingle-lock forming area. From the foregoing description, however, itwill become clear to those of ordinary skill in the thread rolling artthat a wide variety of self-locking characteristics may be produced byobvious extensions of the present teaching as contained in theillustrative embodiments. For example, by providing lock forming areasin a single die at regular intervals it is possible to produce aself-locking screw having a plurality of spaced-apart locking zones.Self-locking action of such a screw may be obtained in a standard nutplaced at any random position along the length of the screw simply byforming the individual locking zones closer together on the screw thanthe thickness of the nut. Other expedients which will become apparent tothose skilled in the art is that combinations of lock forming areas inboth dies either positioned so as to coact in the formation of a singlelocking zone of greater angular extent about the axis of the screw or toform separate locking zones spaced from one another along the length ofthe screw and staggered from side to side. Yet another expedient whichwill become obvious is that a lock forming area may be produced by theremoval of more than one adjacent ridges on a single rolling die oralternatively by grinding away the flanks of more than one adjacentridge. It is accordingly not intended that the scope of the inventionshould be limited to the specific illustrative embodiments hereindescribed but rather that it be interpreted in terms of the appendedclaims.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patents of the United States is 1. In the method ofrolling threads on a screw blank wherein the blank is rolled between tworelatively movable dies each having thread forming working surfaces,which include as thread forming surfaces a series of substantiallyparallel thread root forming ridges and thread crest forming groovesjoined by flank forming walls disposed and spaced at anglescorresponding to the particular thread helix angle to be formed, saidthread forming surface including a locking zone forming area having athread root forming ridge removed to adjacent crest forming grooves, thesteps of first rolling said blank in said dies beginning uniform,continuous helical threads on said blank having a root and a crestjoined by flank portions,

and subsequently rolling said blank in said dies completing said threadstherein and forming a locking zone in a portion of said thread byrolling two adjacent threads only on opposite outer flanks and not onadjacent inner flanks, wherein said adjacent inner flanks areunsupported in said subsequent rolling, thereby deforming the threaddefined between said supported and unsupported flanks by shallowing theroot between said adjacent flanks and steepening the slope of saidadjacent flanks.

2. A die for rolling threads on a screw blank wherein the threads formedinclude a locking zone comprising a base portion and a thread fonningsurface on said base portion, said thread forming surface having aseries of substantially parallel thread root forming ridges and threadcrest forming grooves joined by flank forming walls disposed and spacedat angles corresponding to the particular thread helix angle to befonned and a locking zone forming area adjacent the screw exit end ofsaid thread forming surface, said forming area including a pair ofparallel thread root forming ridges separated by a region having athread root forming ridge removed to adjacent thread crest forminggrooves whereby two adjacent threads in said lock forming area arerolled only on opposite outer flanks and not on adjacent inner flanks bysaid flank forming walls, deforming said threads between said parallelroot forming ridges toward said adjacent flanks.

3. A die according to claim 2 in which the removed ridge cross sectionis removed for the entire length of the die.

1. In the method of rolling threads on a screw blank wherein the blankis rolled between two relatively movable dies each having thread formingworking surfaces, which include as thread forming surfaces a series ofsubstantially parallel thread root forming ridges and thread crestforming grooves joined by flank forming walls disposed and spaced atangles corresponding to the particular thread helix angle to be formed,said thread forming surface including a locking zone forming area havinga thread root forming ridge removed to adjacent crest forming grooves,the steps of first rolling said blank in said dies beginning uniform,continuous helical threads on said blank having a root and a crestjoined by flank portions, and subsequently rolling said blank in saiddies completing said threads therein and forming a locking zone in aportion of said thread by rolling two adjacent threads only on oppositeouter flanks and not on adjacent inner flanks, wherein said adjacentinner flanks are unsupported in said subsequent rolling, therebydeforming the thread defined between said supported and unsupportedflanks by shallowing the root between said adjacent flanks andsteepening the slope of said adjacent flanks.
 2. A die for rollingthreads on a screw blank wherein the threads formed include a lockingzone comprising a base portion and a thread forming surface on said baseportion, said thread forming surface having a series of substantiallyparallel thread root forming ridges and thread crest forming groovesjoined by flank forming walls disposed and spaced at anglescorresponding to the particular thread helix angle to be formed and alocking zone forming area adjacent the screw exit end of said threadforming surface, said forming area including a pair of parallel threadroot forming ridges separated by a region having a thread root formingridge removed to adjacent thread crest forming grooves whereby twoadjacent threads in said lock forming area are rolled only on oppositeouter flanks and not on adjacent inner flanks by said flank formingwalls, deforming said threads between said parallel root forming ridgestoward said adjacent flanks.
 3. A die according to claim 2 in which theremoved ridge cross section is removed for the entire length of the die.